One way of producing fuel is to gasify a carbonaceous raw material, wherein it becomes gas which can be burnt. Particularly popular raw materials include waste materials which are normally organic materials, such as various types of wood waste and package waste (for example, cardboard and paper). The material is also called recovered fuel.
The gasification normally takes place in a fluidized bed process at a high temperature under air-deficient conditions. The product gas obtained is suitable to be burned in, for example, a conventional power plant boiler.
Before its burning, the product gas produced in gasification has to be purified by removing various solids and other impurities. So-called fly ash separated from the product gas contains most of the heavy metals, chlorine and alkali metals of the raw material. The composition of the ash will depend on the raw materials gasified.
The ash contains soluble components which make it hazardous waste. Disposal sites are provided for ash fraction classified as hazardous waste. However, the ash obtained in purifying the gas produced by a gasifier has a high content of soot (10 to 40%) which complicates the final disposal of the ash, because the carbon content exceeds the allowed limit. Such ash can be subjected to special treatment at a hazardous waste disposal plant, but this is relatively expensive.
Finnish patent Fl 110266 discloses a method for treating soot-containing solids of the product gas from gasification already at the gasification plant. In this method, ash having a high soot content and being separated from the product gas flow is introduced in a fluidized bed reactor acting as an oxidizer where it is burnt at a temperature of 800 to 900° C. to oxidize the carbon to carbon dioxide. The combustion flue gases are redirected as secondary gasification gas to the gasifier. Before this, ash is separated from the flue gases.
However, such a solution to recirculate the flue gases is problematic, because the additional gas flow from the oxidization is rich in oxygen, which makes the control of the gasification process more difficult. Also, the capacity diminishes, because part of the capacity is taken by the flue gas flow which increases an inert flow through the gasifier.